The following acronyms are defined in this specification:
A/D analog-to-digital
AGC automatic gain control circuit
CAP constant average power
CD compact-disc data-storage drives.
DCC DC control
DML differential multilevel
DV digital value
DSS digital sum square
EFM eight-fourteen modulation
ISI intersymbol interference
PR1 partial-response class 1
RAP The running average power
RLL run-length limit
RRTP residual running total power
RTP running total power
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. Note that in this specification, the term multilevel refers to more than two levels. In a traditional optical recording system, reflectivity of the recording medium is modulated between two states. The density of data recorded on an optical recording medium can be increased by modulating the reflectivity of the optical recording medium into more than two states. U.S. Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter “Kobayashi”) discloses a system for recovering multilevel data from an optical disc.
FIG. 1 is a block diagram illustrating the system disclosed in Kobayashi for recovering such data. Analog data read from a detector is input from a mark-length detection circuit 101 and a reflectivity detection circuit 102. The outputs of these circuits are sent to an analog-to-digital (A/D) converter 103. The A/D converter 103 includes an n-value circuit 104 which determines the value to which the signal corresponds by comparing the signal to predetermined reference voltages. Subsequently, the n-value signal is converted into a binary signal by binary circuit 105. While this system discloses the concept of reading a multilevel signal and converting it into a digital signal in a basic sense, no method is disclosed for handling various imperfections in a multilevel signal read from an optical disc that tend to occur in practice. Specifically, envelope fluctuations in the multilevel signal can significantly degrade the performance of subsequent detection circuits. These envelope fluctuations are a form of amplitude modulation of the signal caused by variations in the characteristics of the optical disc or in the drive mechanism that are separate from the multilevel modulation.
For example, variations in the index of refraction or the thickness of the polycarbonate material covering the surface of the disc cause distortions. Also, disc warpage may cause variations in both the recorded and readback signals because, as the disc spins, different portions of the disc come into and out of focus. Envelope fluctuations can be separated into two components: a common-mode component, or DC offset; and a differential-mode component. These envelope fluctuations are often called snaking of the signal, which alludes to the snake-like visual appearance of such a signal plotted over a long period of time. The process of removing such fluctuations, particularly the differential-mode component, is called desnaking.
Simple high-pass filtering, such as AC-coupling, can remove the common-mode component. An automatic gain control (AGC) circuit can desnake the differential-mode component of the signal. FIG. 2A is a block diagram of a typical analog AGC circuit for removing envelope fluctuations. The analog input signal enters a variable-gain amplifier 200. The amplifier output enters an analog envelope or average-power detector 202, which feeds back information on the current envelope or average-power level to control the variable-gain amplifier in order to maintain a relatively constant output signal envelope. Analog AGC circuits are commonly used in communications and data-storage systems. Such circuits are inexpensive and easy to manufacture. However, they normally achieve only coarse adjustment of envelope fluctuations. When finer adjustment is needed, a digital AGC circuit, in addition to or in place of an analog AGC circuit, can be used.
FIG. 2B is a block diagram of a digital AGC circuit. The analog input signal first enters an A/D converter 210. The converted digital signal then enters a digital variable-gain amplifier 212. The amplifier output enters a digital envelope or average-power detector 214, which feeds back information on the current envelope or average-power level to control the variable-gain amplifier in order to maintain a relatively constant output signal envelope. Alternatively, the A/D converter and variable-gain amplifier can be combined together, so that the feedback signal directly controls the gain and offset of the A/D converter. Both analog and digital AGC circuits use either an envelope or average-power detector, which requires that the original signal either reach the envelope extrema (both maximum and minimum envelope levels) or have constant average power, respectively, over the time scale of the feedback loop.
Binary signals in data-storage systems, whether magnetic, optical, or magneto-optic, all have run-length limit (RLL) and DC control (DCC) coding, such as eight-fourteen modulation (EFM) in compact-disc (CD) data-storage drives. Consequently, the binary signals written to the disc have constant average power and, even in the presence of intersymbol interference (ISI), will reach envelope extrema frequently.
Multilevel signals, however, are not guaranteed to ever reach their extrema because the signals can stay within middle levels indefinitely. Even when extreme levels do occur, if ISI is present, the multilevel data signal must stay at that maximum or minimum level for several marks for the read signal to reach the envelope extreme. Moreover, the average power of a multilevel signal, even with DCC, is not necessarily constant. In order for a multilevel optical read system to reliably desnake a read signal, a method for detecting envelope fluctuations is needed for a multilevel signal with the characteristics described above.